The VPSA and PSA processes employ a selective adsorbent to remove at least one component of a gas from a gas mixture. Both processes employ four basic process steps: adsorption, depressurization, purge and repressurization. PSA and VPSA processes are well known and are widely used to selectively separate air components, i.e. oxygen and nitrogen.
The design of the adsorber vessel is critical to efficient operation of an air separation system. Improvements in the design of the adsorber vessel contribute to lower power consumption, lower capital cost and increased plant capacity.
VPSA vessels are typically designed as axial flow adsorbers which have limited applicability when plant capacity requirements yield a vessel diameter larger than 4–5 meters, thereby exceeding economical shipment limits. This results in an expensive and difficult requirement to field assemble the axial flow vessels. Such large diameter vessels also have inherently large void volume percentages in the upper and lower head spaces, and present flow distribution difficulties as a result of the large cross-section. As a result, the economics of large scale VPSA systems (i.e., >80 tons per day) are compromised when axial flow, vessel designs are employed.
VPSA system operation is adversely affected by bed pressure drop and void volume within the vessel. Bed pressure drop represents a substantial source of inefficiency in a VPSA process. Large gas flows into and out of the adsorbers are required, due to the relatively low operating pressures and recovery of these systems. This large gas flow results in high superficial gas velocities across the bed, creating an unwanted pressure drop, and contributing to a loss in efficiency. Such bed pressure drop losses typically comprise 10–15% of the power consumption.
In an axial flow bed, if the adsorber bed cross-section is increased by enlarging the diameter and lowering the superficial velocity, larger adsorbent inventories are required. This increases capital cost in order to improve power consumption, resulting in little gain in overall economics.
Void spaces in an adsorption vessel also create losses in a VPSA system. The volume of gas left in a lower head space is pressurized and depressurized during the cycle, ultimately resulting in air blow-down losses. Similarly, the volume of gas left in an upper head space, which is enriched in oxygen after the product make step, is subsequently evacuated in the waste step and acts as an inefficient oxygen purge. This inefficient use of oxygen purge gas results in a lowering of the overall process efficiency.
Advanced VPSA cycles employ powerful adsorbents with a relatively short cycle, and the blow-down losses and top head oxygen purge losses can become quite large.
The use of advanced adsorbents and cycles drives VPSA process design towards reduced bed length. The advanced adsorbents typically can operate efficiently with a lower transfer length, hence a vessel design that readily accommodates this feature is attractive. The use of shorter bed lengths with axial vessels is possible, but when large size plants are desired, the diameter of the vessel becomes prohibitively large.
In the modern PSA technology due to increased demand for higher product throughputs, the ratio of bed length to bed frontal area becomes smaller and adsorbent beds go through faster cycles. For the case of a radial bed, increasing the bed height relative to bed length, in this way making the bed taller, is one way to achieve higher product flow. Nowadays, the beds are also being designed with little end space to minimize the unit cost and to increase the efficiency. In such challenging conditions, uniform flow distribution in the adsorbent bed is essential for optimum bed performance since it allows the use of all available separation surfaces in the vessel.
Adsorption vessels are used to capture and reduce the contaminants of chemical components such as carbon dioxide, carbon monoxide, nitrogen, oxygen, water, and hydrocarbons of the feed stream to ppm levels. This is carried out by adsorbing gases on different adsorbents such as alumina, carbon, zeolite, and molecular sieves. The adsorbent is contained in vessel and is alternated between adsorption and purge steps. A vessel has generally the bed at the middle of the vessel, and has two distributors, one on the feed side of the bed and the other on the product side of the bed. Flow maldistribution in adsorption vessels can cause early breakthrough, loss of efficiency, sieve movement and local bed fluidzation. To prevent these problems, uniform flow distribution in an adsorbent bed is essential.
U.S. Pat. No. 5,298,226 relates, in general, to apparatus for providing uniform fluid flow in vessels having packing materials or particulates and, in particular, to apparatus for providing uniform gas flow in pressure swing adsorption vessels.
U.S. Pat. No. 5,759,242 relates to a vessel for use in a pressure swing adsorption gas separation process includes an enclosing wall which defines an enclosed space having a top region and a bottom region. An annular adsorbent bed is positioned within the enclosed space and has a porous outer wall, a porous inner wall and adsorbent material positioned between the walls. The porous outer wall is separated from the enclosing wall to create a gas feed channel therebetween, and the porous inner wall surrounds an inner tank whose wall surface is separated from the porous inner wall and creates a product flow channel therebetween. A gas feed/distribution baffle structure is positioned in the bottom region of the vessel and in fluid communication with the gas feed channel to provide a gas feed thereto. The gas feed enters the gas feed channel and the adsorbent bed via the porous outer wall and in a direction generally radially towards the inner porous wall and product flow channel. A product outlet is positioned in the bottom region and in fluid communication with the product flow channel, for collecting product gas passing thereinto via the porous inner wall from the adsorbent bed. A flexible membrane extends between the porous outer wall and the porous inner wall, at the upper extremities thereof, and is pressurized so as to bear upon the upper surface of the adsorbent material to prevent fluidization during the flow.
U.S. Pat. No. 5,716,427 relates to equipment, for example of the PSA type, comprising gas circulation elements for passing the gas horizontally through an adsorbent, which comprise, on at least one vertical side of the adsorbent, a gas distribution volume comprising a first subvolume adjacent to the adsorbent, and a second subvolume separated from the first subvolume by a wall provided with passages having cross-sections and/or a distribution which are selected so as to reduce the variations in a local flow rate along the adsorbent. The equipment is particularly useful in separating gases from air.
U.S. Pat. No. 5,814,129 relates to apparatus and method to improve flow of fluid through an annular bed in a radial flow treatment vessel. An elongated annular baffle is disposed in the reactor adjacent the bed to impart generally U-shaped flow to the fluid either prior to entering or after it exits the bed thus achieving an overall serpentine or reverse U-shaped flow pattern as the fluid proceeds from an entry port to an exit port in the reactor. Means are provided in the baffle to permit minor amounts of fluid to bypass the generally U-shaped flow path in order to correct fluid flow maldistribution through the bed that is attributed to frictional pressure drop in the flow channels adjacent to the bed. A vessel can be operated with fluid flow through the vessel in either direction.
U.S. Pat. No. 4,374,094 relates to a radial flow catalytic reactor in which a gravity supported centerpipe is restrained from vertical upward movement due to thermal cycling of the catalyst and the reactor internals by forming the centerpipe to have uniform vertical and radial permeability in a frustoconical configuration. Gravity effect of the catalyst particles forming the bed act along the tapered side of the centerpipe. Additionally improved permeability to radial flow through the uniformly packed catalyst bed, independently of radial resistance to flow through the reactor due to pressure gradient between top and bottom of the vessel is achieved by compensation for differences in radial distance to the centerpipe from the upper portion to the lower portion of the catalyst bed by changes in the permeability of the tapered conical surface of the centerpipe. Uniformity of radial flow through the catalyst bed is assured by forming the conical portion of the centerpipe from rigid screen material so that reaction “dead” spots do not develop adjacent to the centerpipe.
There is a genuine need to find a better way to engender a uniform velocity across the entire section of the process vessels having packing materials or particles. The enhancement of fluid distribution, i.e., improved uniformity, allows for the use of all the available reaction or separation surface area in the process vessels, thereby efficiently increasing the yield of the desired product.
It is an object of the invention to provide an improved radial bed adsorbent vessel capable of achieving an effective uniform flow of gas through the adsorbent bed therein.
It is another object of the invention to provide an improved VPSA vessel capable of achieving substantial uniform velocity of gas through the bed of the vessel.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.